1. Field of the Invention
This invention relates generally to pneumatic controls and more particularly to pneumatic switches and has particular application to pneumatic level switches.
2. Description of the Related Art
Pilot valves are used in a number of different industries, typically to control process conditions or parameters, by controlling the operation of a process control unit, such as a process control valve. Numerous industries utilize process control valves to control the rate of flow of liquid and to control the level of liquid in tanks and vessels. The process control valves in many cases are operated with a control signal, which may be, for example, a pneumatic signal transmitted from a pilot valve. In such a case, the liquid level in a tank or other vessel may be sensed by a liquid level controller, such as a float, displacer or other element, which moves when the liquid reaches a certain level. Movement of the float will actuate a control device, such as a pilot valve, which will send a signal to the process control valve to open or close the process control valve to either allow flow of liquid from the vessel or to prevent flow therefrom.
In the oil and gas industry, liquid level controllers (called a float hereinafter) may be used to control the level of oil, water or other liquids in a tank or other vessel. The float is placed inside the tank. When the liquid level in the tank is such that it engages the float, the float will move as the level of the liquid changes. The changes in the liquid level are transmitted to a pilot valve which will signal the process control valve to either open or close in response to the changing liquid level in the tank which will either allow flow to or stop flow from the tank.
The pilot valve may be a pneumatic pilot valve to which a supply gas is provided. The pilot valve will direct the supply gas to the process control valve depending upon the level of liquid in the tank and the process control valve will open or close depending on the signal received from the pilot valve.
In one type of pilot valve, the pilot valve provides a sudden increase in output pressure, or a sudden decrease, to the process control valve so that the control valve will open fully, or close fully, almost immediately. For example, when the level of liquid in the tank increases and moves the displacer to a predetermined level, there will be a sudden increase in output pressure from the pilot valve, which will cause the process control valve to move almost immediately from the fully closed to a fully open position to discharge liquid from the tank. When the liquid level falls to a preselected lowermost level, a sudden decrease in output pressure from the pilot valve will occur; this will cause the process control valve to move from the fully open to the fully closed position.
While there are several different pneumatic switches using pilot valves to actuate process control valves, there is a continuing need for new and different pneumatic switches to serve the various needs of industry.